Figure 6 shows the mass number and the atomic number for the nuclei of five different atoms.
i- How many neutrons are there in a nucleus of atom A?
[1 mark]
SOLUTION
The nucleus of atom A has the atomic number and atomic mass as
So, to find neutron number, subtract atomic number from atomic mass.
ii- Which two atoms in Figure 6 are the same element?
[1 mark]
SOLUTION
For the atoms to be of same element, they must have same atomic number. And in the case of this question, atom D & E have the same atomic number, 92.
iii- Nucleus B decays by emitting an alpha particle.
Draw an arrow on Figure 7 to represent the alpha decay.
[2 marks]
SOLUTION
(Since alpha particle has 2 protons and 2 neutrons, the decrease in atomic number of atom B would be by 2 units and decrease in atomic mass of atom B would be 4 units.)
iv- What is meant by the ‘random nature of radioactive decay’?
[1 mark]
ANSWER
Random nature of radioactive decay means unable to predict which atom will decay next (or when it will decay ).
v- A polonium (Po) nucleus decays by emitting an alpha particle and forming a lead (Pb) nucleus.
Po → Pb + α
The lead (Pb) nucleus then decays by emitting a beta particle and forms a bismuth(Bi) nucleus.
Pb → Bi + β
The bismuth (Bi) nucleus then decays by emitting a beta particle and forms a polonium (Po) nucleus.
Bi → Po + β
Explain how these three decays result in a nucleus of the original element, polonium.
[3 marks]
SOLUTION
Initially, the atomic number decreases by 2 as it is undergone alpha decay. But since beta decay occurs twice, therefore the atomic no increases by 2. Therefore the proton/atomic number is the same as the original element.
(This is a sequential decay process, in which polonium nucleus is eventually regenerated.
In the first step, Po → Pb + α, the atom losses 2 protons and 2 neutrons.
Whereas, in the second decay Pb → Bi + β, which is a beta decay, eth neutron turns into a proton , meaning thereby the atomic mass remains the same but atomic number increases by 1.
And, the same happens in the second beta decay, increasing the atomic number.
Thus, the two beta decays cancels out the increase of atomic number in the first alpha decay.)
